U.S. patent number 7,842,381 [Application Number 12/173,802] was granted by the patent office on 2010-11-30 for thermally conductive emi shield.
This patent grant is currently assigned to Laird Technologies, Inc.. Invention is credited to Richard Norman Johnson.
United States Patent |
7,842,381 |
Johnson |
November 30, 2010 |
Thermally conductive EMI shield
Abstract
Electromagnetic-energy absorbing materials are combined with
thermally conductive materials, such as those used for thermal
management in association with electronic equipment, thereby
suppressing the transmission of electromagnetic interference (EMI)
therethrough. Disclosed are materials and processes for combining
EMI-absorbing materials with thermally conductive materials thereby
improving EMI shielding effectiveness in an economically efficient
manner. In one embodiment, a thermally conductive EMI absorber is
prepared by combining an EMI-absorbing material (for example,
ferrite particles) with a thermally conducting material (for
example, ceramic particles), each suspended within an elastomeric
matrix (for example, silicone). In application, a layer of
thermally conductive EMI-absorbing material is applied between an
electronic device or component, and a heat sink.
Inventors: |
Johnson; Richard Norman
(Encinitas, CA) |
Assignee: |
Laird Technologies, Inc.
(Chesterfield, MO)
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Family
ID: |
32176482 |
Appl.
No.: |
12/173,802 |
Filed: |
July 15, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090016025 A1 |
Jan 15, 2009 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10531890 |
Oct 27, 2009 |
7608326 |
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PCT/US03/33353 |
Oct 21, 2003 |
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60419873 |
Oct 21, 2002 |
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Current U.S.
Class: |
428/323;
428/500 |
Current CPC
Class: |
H01L
23/3733 (20130101); H01L 23/3737 (20130101); H05K
9/0083 (20130101); H01L 23/552 (20130101); H05K
7/20481 (20130101); Y10T 428/31551 (20150401); Y10T
428/25 (20150115); Y10T 428/31855 (20150401); Y10T
428/256 (20150115); Y10T 428/257 (20150115); H01L
2224/32245 (20130101); Y10T 428/26 (20150115); Y10T
428/31801 (20150401); Y10T 428/32 (20150115); H01L
2924/16152 (20130101); H01L 2224/73253 (20130101); H01L
2924/01068 (20130101); H01L 2924/01055 (20130101); H01L
2924/3011 (20130101); H01L 2924/16152 (20130101); H01L
2224/73253 (20130101) |
Current International
Class: |
B32B
5/16 (20060101); B32B 27/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1359989 |
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Jul 2002 |
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CN |
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0945916 |
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Sep 1999 |
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EP |
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1267601 |
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Dec 2002 |
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EP |
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2001348542 |
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Dec 2001 |
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JP |
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2001358265 |
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Dec 2001 |
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JP |
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2002217342 |
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Aug 2002 |
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JP |
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WO 02/13315 |
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Feb 2002 |
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WO |
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Other References
Research Progress of the Materials for Radar Absorbing Coatings,
Wang Jieliang et al., Modern Paint & Finishing, Feb. 28, 2002,
4 pages. cited by other.
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Primary Examiner: Ahmed; Sheeba
Attorney, Agent or Firm: Harness, Dickey & Pierce,
P.L.C.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional U.S. patent application Ser. No.
10/531,890 filed Nov. 28, 2005 (now U.S. Pat. No. 7,608,326, issued
Oct. 27, 2009), which, in turn, is a U.S. national stage filing
under 35 U.S.C. 371 of International Application No.
PCT/US2003/33353 filed Oct. 21, 2003 (PCT Publication No.
WO2004/037447published May 6, 2004) which, in turn, claims the
benefit of U.S. provisional patent application No. 60/419,873 filed
Oct. 21, 2002.
Claims
What is claimed is:
1. A thermally conductive composite material for reducing
electromagnetic emissions generated by an electronic device, the
thermally conductive composite material comprising a thermally
conductive material in particulate form, an
electromagnetic-energy-absorptive material including iron silicide
in particulate form, and a polymeric base material, the thermally
conductive material and the electromagnetic-energy-absorptive
material being suspended within the polymeric base material, the
polymeric base material being substantially transparent to
electromagnetic energy, wherein the thermally conductive composite
material is configured such that when placed between an electronic
device and a proximate structure, the thermally conductive material
is operable for facilitating transfer of thermal energy from the
electronic device and the electromagnetic-energy-absorptive
material is operable for reducing electromagnetic emissions
generated by the electronic device.
2. A thermally conductive composite material as claimed in claim 1
wherein the electromagnetic-energy-absorptive material comprises
generally ellipsoidal iron silicide granules.
3. A thermally conductive composite material as claimed in claim 1
wherein the thermally conductive material includes at least one of
aluminum nitride, boron nitride, iron, metallic oxides and
combinations thereof.
4. A thermally conductive composite material as claimed in claim 1
wherein the polymeric base material comprises a phase-change
material configured to exist in a solid phase at ambient room
temperature and transition to a liquid phase at a reflow
temperature to conform to a surface of a device.
5. A thermally conductive composite material as claimed in claim 1
wherein the electromagnetic-energy-absorptive material is entirely
iron silicide.
6. A thermally conductive composite material as claimed in claim 1
wherein at least one of the thermally conductive material and the
electromagnetic-energy-absorptive material comprises particles in
the form of granules having a spheroid shape.
7. A thermally conductive composite material as claimed in claim 1
wherein the thermally conductive material comprises a ceramic
material.
8. A thermally conductive composite material as claimed in claim 1
wherein the composite material is in the form of a sheet, and
further comprises an adhesive on at least one side of the
sheet.
9. A thermally conductive composite material as claimed in claim 8
wherein the adhesive comprises at least one or more of: a
thermoconductive adhesive, or a pressure-sensitive, thermally
conductive adhesive, or acrylic, or silicone, or rubber, or ceramic
powder, or any combination thereof.
10. A thermally conducting composite material as claimed in claim 1
wherein the polymeric base material has a relative dielectric
constant of less than approximately 4 and a loss tangent of less
than approximately 0.1, whereby the polymeric base material does
not impede the absorbtive action of the
electromagnetic-energy-absorptive material.
11. A thermally conductive composite material as claimed in claim 1
wherein the polymeric base material comprises at least one or more
of: a solid thermoplastic material, or a solid thermosetting
material, or a mixture of a paraffin wax and an ethylene-vinyl
acetate copolymer, or a synthetic wax having a melting point of
approximately 100.degree. C. and a molecular weight of
approximately 1000, or an elastomer, or a natural rubber, or a
synthetic rubber, or PDP, or EPDM rubber, or silicone, or
fluorosilicone, or isoprene, or nitrile, or chlorosulfonated
polyethylene, or neoprene, or fluoroelastomer, or urethane, or
thermoplastic, or thermoplastic elastomer (TPE), or polyamide TPE,
or thermoplastic polyurethane (TPU), or any combination
thereof.
12. A thermally conductive composite material as claimed in claim 1
wherein the polymeric base material is a liquid.
13. A thermally conductive composite material as claimed in claim 1
wherein the electromagnetic-energy-absorptive material has a
relative magnetic permeability greater than about 3.0 at
approximately 1.0 GHz and greater than about 1.5 at 10 GHz.
14. A thermally conductive composite material as claimed in claim
13 wherein the liquid comprises at least one or more of: silicone,
or epoxy, or polyester resin, or any combination thereof.
15. A thermally conductive composite material as claimed in claim 1
wherein the composite material is in the form of a sheet having a
thickness greater than approximately 0.01 inches and less than
approximately 0.18 inches.
16. A thermally conductive composite material as claimed in claim 1
wherein: the electromagnetic-energy-absorptive material exhibits
better thermal conductivity than air; and the thermally conductive
material exhibits greater thermal conductivity than the
electromagnetic-energy-absorptive material, the thermally
conductive material having a thermal impedance value substantially
less than that of air.
17. A thermally conductive composite material as claimed in claim 1
wherein the composite material includes about 60 percent by volume
of the thermally conductive material and the
electromagnetic-energy-absorptive material.
18. A thermally conductive composite material as claimed in claim 1
wherein: the composite material is in the form of a sheet having a
thickness of about 0.125 inch and exhibits an attenuation of at
least about 5 dB in a frequency range from about 5 GHz up to at
least about 18 GHz; or the composite material is in the form of a
sheet having a thickness of about 0.02 inch and exhibits an
attenuation of at least about 3 dB for a frequency range extending
upward from about 10 GHz; or the composite material is in the form
of a sheet having a thickness of about 0.04 inch and exhibits an
attenuation of at least about 10 dB in a frequency range from about
9 GHz up to at least about 15 GHz and an attenuation of at least
about 6 dB in a frequency range extending upward from about 15 GHz;
or the composite material is in the form of a sheet having a
thickness of about 0.060 inch and exhibits an attenuation of at
least about 5 dB in a frequency range extending upward from about 4
GHz, having a greater attenuation of at least about 10 dB in a
frequency range from about 6 GHz up to at least about 10 GHz.
19. A thermally conductive composite material as claimed in claim 1
wherein: the thermally conductive material in particulate form
comprises granules spaced-apart from each other; the
electromagnetic-energy-absorptive material in particulate form
comprises granules spaced apart from each other and spaced-apart
from the granules of the thermally conductive material; and the
composite material is electrically non-conductive.
20. A thermally conductive composite material as claimed in claim 1
wherein the thermally conductive material comprises
microspheres.
21. An electronic component comprising an integrated circuit, a
heat sink, and the composite material of claim 1.
22. A method of reducing electromagnetic emissions produced by a
device, the method comprising: suspending a thermally conductive
material in particulate form and an
electromagnetic-energy-absorptive material including iron silicide
in particulate form in a polymeric base material; and placing the
thermally conductive material and electromagnetic-energy-absorptive
material suspended in the polymeric base material between the
device and a proximate structure.
23. The method of claim 22 wherein: the thermally conductive
material and electromagnetic-energy-absorptive material suspended
in the polymeric base comprise a liquid solution; and placing
comprises applying the liquid solution onto one or more surfaces of
at least one of the device and the proximate structure.
24. The method of claim 22 wherein: the thermally conductive
material and electromagnetic-energy-absorptive material suspended
in the polymeric base comprise a liquid solution; placing comprises
applying the liquid solution onto one or more surfaces of at least
one of the device and the proximate structure that have one or more
surface imperfections, and allowing the liquid solution to flow
into the one or more surface imperfections.
25. The method of claim 22 wherein: the thermally conductive
material and electromagnetic-energy-absorptive material suspended
in the polymeric base comprise a liquid solution; and placing
comprises spraying the liquid solution onto one or more surfaces of
at least one of the device and the proximate structure.
26. The method of claim 22 wherein: the thermally conductive
material and electromagnetic-energy-absorptive material suspended
in the polymeric base comprise a liquid solution; and placing
comprises painting the liquid solution onto one or more surfaces of
at least one of the device and the proximate structure.
27. The method of claim 22 further comprising combining the
thermally conductive material in particulate form with the
electromagnetic-energy-absorptive material in particulate form
before suspending within the polymeric base.
28. The method of claim 22 wherein the proximate structure
comprises a heat sink, and wherein the method includes placing the
thermally conductive material and electromagnetic-energy-absorptive
material suspended in the polymeric base material between the
device and the heat sink.
29. The method of claim 22 wherein the device comprises an
integrated circuit, and wherein the method includes placing the
thermally conductive material and electromagnetic-energy-absorptive
material suspended in the polymeric base material between the
integrated circuit and the proximate structure.
30. A thermally conductive composite material for reducing
electromagnetic emissions generated by an electronic device, the
thermally conductive composite material comprising a thermally
conductive material in particulate form, an
electromagnetic-energy-absorptive material including iron silicide
in particulate form, and a polymeric base material, the thermally
conductive material and the electromagnetic-energy-absorptive
material being suspended within the polymeric base material, the
polymeric base material being substantially transparent to
electromagnetic energy, and comprising a phase-change material
including a mixture of a paraffin wax and an ethylene-vinyl acetate
copolymer, which is configured to exist in a solid phase at ambient
room temperature and transition to a liquid phase at a reflow
temperature, to conform to a surface of a device, wherein the
thermally conductive composite material is configured such that
when placed between an electronic device and a proximate structure,
the thermally conductive material is operable for facilitating
transfer of thermal energy from the electronic device and the
electromagnetic-energy-absorptive material is operable for reducing
electromagnetic emissions generated by the electronic device.
31. A thermally conductive composite material for reducing
electromagnetic emissions generated by an electronic device, the
thermally conductive composite material comprising: a thermally
conductive material in particulate form; and an
electromagnetic-energy-absorptive material in particulate form, the
thermally conductive material and the
electromagnetic-energy-absorptive material being suspended within a
matrix material that is conformable, even after being heated, to
surface imperfections of a mating surface; wherein the thermally
conductive composite material is configured such that when placed
between an electronic device and a proximate structure, the
thermally conductive material is operable for facilitating transfer
of thermal energy from the electronic device and the
electromagnetic-energy-absorptive material is operable for reducing
electromagnetic emissions generated by the electronic device.
32. A thermally conductive composite material as claimed in claim
31, wherein the matrix material comprises a phase-change material
having a reflow temperature that allows the thermally conductive
material in particulate form that is suspended within the
phase-change material to flow into gaps when heated to the reflow
temperature.
33. A thermally conductive composite material as claimed in claim
31, wherein the thermally conductive composite includes up to about
60% by volume of the thermally conductive material and the
electromagnetic-energy-absorbtive material in particulate form
suspended in the matrix material without compromising
conformability of the matrix material.
34. A thermally conductive composite material as claimed in claim
31, wherein: the electromagnetic-energy-absorptive material
includes carbonyl iron and/or iron silicide; and/or the matrix
material comprises a mixture of 25 parts by weight of a paraffin
wax and 6 parts by weight of an ethylene-vinyl acetate copolymer,
or a mixture of 95 parts by weight of a paraffin wax and 5 parts by
weight of an ethylene-vinyl acetate copolymer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to thermal management in
electronic applications and, more specifically, to thermal
conductors incorporating electromagnetic-energy-attenuating
properties.
2. Description of the Prior Art
As used herein, the term EMI should be considered to refer
generally to both electromagnetic interference and
radio-frequency-interference (RFI) emissions, and the term
"electromagnetic" should be considered to refer generally to
electromagnetic and radio frequency.
Electronic devices typically generate thermal emissions as an
unavoidable byproduct. The amount of thermal emissions generated
can correlate to the switching speed and complexity of the source
electronic component or device. As newer electronic devices tend to
operate at greater and greater switching speeds, they will also
result in greater thermal emissions. These increased thermal
emissions, at some level, pose a risk of interfering with the
function of the source electronic component, and with the functions
of other nearby devices and components.
Accordingly, the unwanted thermal emissions should be dissipated
benignly to preclude or minimize any undesirable effects. Prior-art
solutions addressing the removal of unwanted thermal emissions
include providing a thermal pad over the electronic component and
attaching a heat sink to the thermal pad. Heat sinks generally
include material with high thermal conductivity. When placed in
intimate contact with a heat-generating electronic component, the
heat sink conducts thermal energy away from the component. Heat
sinks also include attributes that facilitate heat transfer from
the heat sink to the ambient environment, for example, through
convection. For example, heat sinks often include "fins" that
result in a relatively large surface area for a given volume.
Furthermore, under normal operation, electronic equipment typically
generates undesirable electromagnetic energy that can interfere
with the operation of proximately located electronic equipment due
to EMI transmission by radiation and conduction. The
electromagnetic energy can exist over a wide range of wavelengths
and frequencies. To minimize problems associated with EMI, sources
of undesirable electromagnetic energy can be shielded and
electrically grounded to reduce emissions into the surrounding
environment. Alternatively, or additionally, susceptors of EMI can
be similarly shielded and electrically grounded to protect them
from EMI within the surrounding environment. Accordingly, shielding
is designed to prevent both ingress and egress of electromagnetic
energy relative to a barrier, a housing, or other enclosure in
which the electronic equipment is disposed.
Sound EMI design principles recommend that EMI be treated as near
as possible to the source to preclude entry of unwanted EMI into
the local environment, thereby minimizing the risk of interference.
Unfortunately, components and devices requiring the use of heat
sinks are not well suited for protective treatment for EMI at the
source, because such treatment would interfere with the operation
of the heat sink. The heat sink should be in intimate contact with
the electronic component to provide a thermal conduction path and
also be open to the surrounding environment to allow for the heat
sink to function through convective heat transfer.
SUMMARY OF THE INVENTION
In general, the present invention relates to an
electromagnetic-interference-absorbing thermally-conductive gap
filler, such as an elastomeric (for example, silicone) pad treated
with an electromagnetic-interference-absorbing material. The
EMI-absorbing material absorbs a portion of the EMI incident upon
the treated thermal pad, thereby reducing transmission of EMI
therethrough over a range of operational frequencies. The absorbing
material may remove a portion of the EMI from the environment
through power dissipation resulting from loss mechanisms. These
loss mechanisms include polarization losses in a dielectric
material and conductive, or ohmic, losses in a conductive material
having a finite conductivity.
Accordingly, in a first aspect, the invention relates to a
composite material for reducing electromagnetic emissions generated
by an electronic device, the composite material including, in
combination, a thermally conductive material and an
electromagnetic-energy-absorptive material. The thermally
conductive material facilitates transfer of thermal energy from the
device and the electromagnetic-energy-absorptive material reduces
electromagnetic emissions generated by the device.
In one embodiment, at least one of the thermally conductive
material and the electromagnetic-energy-absorptive material are
granules. The granules may be generally spherical, such as
microspheres, or other shapes, such as powder, fibers, flakes, and
combinations thereof. The composite further includes a matrix
material in which the thermally conductive material and the
electromagnetic-energy-absorptive material are suspended.
In general, the matrix material is substantially transparent to
electromagnetic energy, for example, being defined by a relative
dielectric constant of less than approximately 4 and a loss tangent
of less than approximately 0.1. In one embodiment, the matrix is
prepared as a liquid. In another embodiment, the matrix is prepared
as a solid. In another embodiment, the matrix is prepared as a
phase-change material existing in a solid phase at ambient room
temperature and transitioning to a liquid phase at
equipment-operating temperatures. In another embodiment, the matrix
is prepared as a thermosetting material.
In some embodiments, the thermally conductive EMI absorber is
formed in a sheet having a thickness greater than approximately
0.010 inch and less than approximately 0.18 inch. In other
embodiments, the sheet includes a thermoconductive adhesive
layer.
In another aspect, the invention relates to a method for reducing
electromagnetic emissions produced by a device, the method
including the steps of providing a thermally conductive material,
providing an electromagnetic-absorbing material, and combining the
thermally conductive material with the electromagnetic-absorbing
material.
In one embodiment, the process includes the additional step of
suspending the combined thermally conductive material and
electromagnetic-absorbing material in a matrix material.
In another embodiment, the process includes the additional step of
placing the combined thermally conductive material and
electromagnetic-absorbing material between the device and proximate
structure, such as between an integrated circuit and a heat
sink.
Further areas of applicability will become apparent from the
description provided herein. It should be understood that the
description and specific examples are intended for purposes of
illustration only and are not intended to limit the scope of the
present disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
The advantages of the invention may be better understood by
referring to the following description, taken in conjunction with
the accompanying drawings, in which:
FIG. 1 is a schematic diagram depicting a perspective view of an
embodiment of a thermally conductive EMI absorber identifying
exemplary constituent components;
FIG. 2 is a schematic diagram depicting a perspective view of an
exemplary application of a thermally conductive EMI absorber, such
as the embodiment illustrated in FIG. 1;
FIGS. 3A and 3B are schematic diagrams depicting perspective views
of alternative embodiments of a thermally conductive EMI absorber
formed as a sheet and as a rollable tape, respectively;
FIG. 4 is a schematic diagram depicting an alternative embodiment
of the thermally conductive EMI absorber depicted in FIG. 1, in
which desired shapes are cut, for example, from the sheet of FIG.
3A;
FIG. 5 is a schematic diagram depicting a perspective view of an
alternative embodiment of the thermally conductive EMI absorber
depicted in FIG. 1, in which the shield is pre-formed according to
a predetermined shape;
FIG. 6 is a schematic diagram of an alternative embodiment of a
thermally conductive EMI absorber in a flowable form, such as a
liquid;
FIG. 7 is a schematic diagram depicting a perspective view of an
exemplary application of a flowable, thermally conductive EMI
absorber, such as the embodiment illustrated in FIG. 6;
FIG. 8 is a flow diagram depicting an embodiment of a process for
preparing a thermally conductive EMI absorber, such as the
embodiment illustrated in FIG. 1; and
FIG. 9 is a schematic plan view of a test fixture used to measure
the thermal conductivity of the thermally conductive EMI shield of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Materials having electromagnetic-energy absorbing properties can be
used to suppress the transmission of EMI over a broad range of
frequencies. Such EMI-absorbing materials can provide substantial
electromagnetic-shielding effectiveness, for example, up to about 5
dB or more at EMI frequencies occurring from about 2 GHz up to
about 100 GHz.
According to the present invention, a thermally-conductive EMI
absorber can be formed by combining EMI-absorbing fillers and
thermally conducting fillers in a base matrix (for example, an
elastomer) capable of being applied as a thermal gap filler, or
pad. Generally, the resulting thermally-conductive EMI absorber can
be applied as any thermal conductive material, for example, as
between an electronic component (e.g., a "chip") and a heat
sink.
Referring to FIG. 1, a thermally-conductive EMI absorber (thermal
EMI shield) 100 is illustrated as a rectangular volume. The front
face of the thermal EMI shield 100 represents a cross-sectional
view of the interior composition of the shield 100. Namely, the
thermal EMI shield 100 includes a number of EMI absorbers 110 and a
number of thermal conductors 120, both being suspended within a
matrix material 130. Although none of the EMI absorber particles
110 and the thermal conductor particles 120 are illustrated as
being in contact with any neighboring particles 110, 120,
configurations in which such contact occurs are anticipated. For
example, thermal conductivity of the thermal EMI shield 100 would
generally be enhanced for configurations in which thermal conductor
particles 120 are in close proximity and contact with each
other.
The relative sizes of the individual EMI absorbers 110, thermal
conductors 120, and the thickness of the matrix 130 as shown in
FIG. 1 are for illustration purposes only. In general, the
suspended fillers 110, 120 are extremely small (that is,
microscopic). Small filler particles 110, 120 allow for embodiments
in which the overall thickness of the thermal EMI shield 100 is
thin, for example, the thickness of the thermal EMI shield 100 is
substantially less than the thickness of either the electronic
component/device or the heat sink.
Similarly, the relative shapes of the suspended particles 110, 120
can be any arbitrary shape. The elliptical shapes of the suspended
particles 110, 120 shown in FIG. 1 are for illustration purposes
only. In general, the shape of the suspended particles 110, 120 can
be granules, such as spheroids, ellipsoids, or irregular spheroids.
Alternatively, the shape of the suspended particles 110, 120 can be
strands, flakes, a powder, or combinations of any or all of these
shapes.
The EMI absorbers 110 function to absorb electromagnetic energy
(that is, EMI). Specifically, the EMI absorbers 110 convert
electromagnetic energy into another form of energy through a
process commonly referred to as a loss. Electrical loss mechanisms
include conductivity losses, dielectric losses, and magnetization
losses. Conductivity losses refer to a reduction in EMI resulting
from the conversion of electromagnetic energy into thermal energy.
The electromagnetic energy induces currents that flow within an EMI
absorber 110 having a finite conductivity. The finite conductivity
results in a portion of the induced current generating heat through
a resistance. Dielectric losses refer to a reduction in EMI
resulting from the conversion of electromagnetic energy into
mechanical displacement of molecules within an absorber 110 having
a non-unitary relative dielectric constant. Magnetic losses refer
to a reduction in EMI resulting from the conversion of
electromagnetic energy into a realignment of magnetic moments
within an EMI absorber 110.
In some embodiments, the EMI absorber 110 exhibits better thermal
conductivity than air. For example, spherical iron particles
selected as an EMI absorber 110 because of their EMI-absorbing
properties also offer some level of thermal conductivity.
Generally, however, the thermal conductivity of the EMI absorbers
110 of comparable thicknesses is substantially less than the value
of thermal conductivity offered by substantially non-EMI-absorbing
thermal conductors 120, such as ceramic particles.
In general, the EMI absorber 110 is selected from the group
consisting of electrically conductive material, metallic silver,
carbonyl iron powder, SENDUST (an alloy containing 85% iron, 9.5%
silicon and 5.5% aluminum), ferrites, iron silicide, magnetic
alloys, magnetic flakes, and combinations thereof. In some
embodiments, the EMI absorber 110 is a magnetic material. In one
particular embodiment, the EMI absorber 110 has a relative magnetic
permeability greater than about 3.0 at approximately 1.0 GHz, and
greater than about 1.5 at 10 GHz.
The thermal conductor 120 includes a thermal impedance value
substantially less than that of air. A low value of thermal
impedance allows the thermal conductor 120 to efficiently conduct
thermal energy. In general, the thermal conductor 120 is selected
from the group consisting of aluminum nitride (AIN), boron nitride,
iron (Fe), metallic oxides and combinations thereof. In some
embodiments, the thermal conductor includes a ceramic material. In
one particular embodiment, the thermal conductor 120 includes a
Fe--AIN (40% and 20% by volume, respectively) having a thermal
conductivity value greater than about 1.5 Watts/m-.degree. C. An
exemplary test report including a test procedure for measuring the
thermal conductivity of a test sample, as well as measured thermal
conductivity test results, is provided herein as Appendix A and
incorporated herein in its entirety.
In general, the matrix material 130 is selected to have properties
allowing it to conform to surface imperfections encountered in many
heat-sink applications (for example, surface imperfections of the
mating surfaces of either the electronic component or device and
the heat sink). Other desirable properties of the matrix material
130 include an ability for the material 130 to accept and suspend a
substantial volume of particles 110, 120, (for example, up to about
60% by volume) without compromising the other advantageous
properties of the matrix material 130, such as conformability,
compliance, and resilience. Generally, the matrix material 130 is
also substantially transparent to electromagnetic energy so that
the matrix material 130 does not impede the absorptive action of
the EMI absorbers 110. For example, a matrix material 130
exhibiting a relative dielectric constant of less than
approximately 4 and a loss tangent of less than approximately 0.1
is sufficiently transparent to EMI. Values outside this range,
however, are also contemplated.
Generally, the matrix material 130 can be selected as a solid, a
liquid, or a phase-change material. Embodiments in which the matrix
material 130 is a solid further include thermoplastic materials and
thermoset materials. Thermoplastic materials can be heated and
formed, then reheated and re-formed repeatedly. The shape of
thermoplastic polymer molecules is generally linear, or slightly
branched, allowing them to flow under pressure when heated above
the effective melting point. Thermoset materials can also be heated
and formed; however, they cannot be reprocessed (that is, made to
flow under pressure when reheated). Thermoset materials undergo a
chemical as well as a phase change when they are heated. Their
molecules form a three-dimensional cross-linked network.
In some solid embodiments, the matrix material 130 is selected from
the group consisting of elastomers, natural rubbers, synthetic
rubbers, PDP, ethylene-propylene diene monomer (EPDM) rubber, and
combinations thereof. In other embodiments the matrix material 130
includes a polymer. The matrix material 130 can also be selected
from the group consisting of silicone, fluorosilicone, isoprene,
nitrile, chlorosulfonated polyethylene (for example,
HYPALON..RTM.), neoprene, fluoroelastomer, urethane,
thermoplastics, such as thermoplastic elastomer (TPE), polyamide
TPE and thermoplastic polyurethane (TPU), and combinations
thereof.
Referring to FIG. 2, an exemplary application is illustrated in
which an electronic component 200, shown mounted on a circuit board
210, is fitted with a heat sink 220. The electronic component 200
can be an electronic circuit (for example, a microcircuit, or
"chip"). Alternatively, the electronic component 200 can be an
electronic device, such as a packaged module including one or more
electronic components (for example, mounted within a metallic
housing, or "can"). In either instance, the electronic component
200 creates, as a byproduct of its electronic function, thermal
energy that should be dissipated to ensure that the electronic
component 200 continues to operate within its design parameters and
is protected from physical damage due to overheating.
In general, a heat sink 220 is a device for dissipating heat from a
host component 200. The heat sink 220 first absorbs heat from the
host component 200 through conduction. The heat sink 220 then
dissipates the absorbed heat through convection to the surrounding
air. The particular type or form of heat sink 220 selected is not
critical. Rather, the heat sink 220 can be any one of a numerous
variety of commercially available heat sinks, or even a custom
designed heat sink.
The thermal EMI shield 230 facilitates thermal conduction from the
component 200 to the heat sink 220. Generally, the thickness of the
thermal EMI shield 230 (the dimension between the protected
component 200 and the heat sink) is less than a predetermined
maximum value. For example, in one embodiment, the thermal EMI
shield 230 has a maximum thickness less than approximately 0.18
inch. Furthermore, the thickness of the thermal EMI shield 230 is
generally greater than a predetermined minimum value. If the
thermal EMI shield is too thin, an insufficient volume of EMI
absorbing material will be provided to sufficiently absorb EMI from
the component 200. For example, in one embodiment, the thermal EMI
shield 230 has a minimum thickness greater than approximately 0.01
inch.
In one exemplary configuration, a thermal EMI shield 230 having a
thickness of 0.125 inch, exhibits an attenuation of at least about
5 dB in a frequency range from about 5 GHz up to at least about 18
GHz. In another exemplary configuration, a thermal EMI shield 230
having a thickness of 0.02 inch, exhibits an attenuation of at
least about 3 dB for a frequency range extending upward from about
10 GHz. In another exemplary configuration, a thermal EMI shield
230 having a thickness of 0.04 inch, exhibits an attenuation of at
least about 10 dB in a frequency range from about 9 GHz up to at
least about 15 GHz and an attenuation of at least about 6 dB in a
frequency range extending upward from about 15 GHz. In yet another
exemplary configuration, a thermal EMI shield 230 having a
thickness of 0.060 inch, .+-.0.005 inch, exhibits an attenuation of
at least about 5 dB in a frequency range extending upward from
about 4 GHz, having a greater attenuation of at least about 10 dB
in a frequency range from about 6 GHz up to at least about 10 GHz.
Exemplary values of the complex (real and imaginary) relative
permittivity (.di-elect cons..sub.r) and complex (real and
imaginary) relative magnetic permeability (.mu..sub.r) for a
nitrile rubber compound are tabulated and provided herein as
Appendix B, incorporated herein in its entirety.
Referring to FIG. 3A, a thermal EMI shield is illustrated in a
sheet configuration. Generally, the thermal EMI shield can be
formed as a sheet 300. The sheet 300 includes a length (L') a width
(W') and a thickness (T'). In one embodiment, the length and width
may be selected according to the dimensions of a particular
application, such as the length and width of an electronic
component 200 to which a heat sink 220 will be applied. In another
embodiment, the sheet 300 can be fabricated in a predetermined
size, such as a length of 26 inches, a width of 6 inches, and a
thickness of either 0.030 inch or 0.060 inch. Any size, however, is
contemplated.
Yet other embodiments of a thermal EMI shield 100 may include a
sheet 300 as just described, further including an adhesive layer
310. The adhesive layer 310 may be a thermoconductive adhesive to
preserve the overall thermal conductivity. The adhesive layer 310
can be used to affix the heat sink 220 to the electronic component
200. In some embodiments, the sheet 300 includes a second adhesive
layer, the two layers facilitating the adherence of the heat sink
220 to the electronic component 200. In some embodiments, the
adhesive layer 310 is formulated using a pressure-sensitive,
thermally-conducting adhesive. The pressure-sensitive adhesive
(PSA) may be generally based on compounds including acrylic,
silicone, rubber, and combinations thereof. The thermal
conductivity is enhanced, for example, by the inclusion of ceramic
powder.
In an alternative embodiment, referring now to FIG. 3B, the thermal
EMI shield may be formed as a tape 320. The tape 320, for example,
can be stored on a roll 330, similar in form to a conventional roll
of adhesive-backed tape. The tape 320 generally exhibits
construction and composition features similar to those already
described in relation to the sheet 300 of FIG. 3A. Similar to the
sheet 300, the tape 320 includes a second width (W'') and a second
thickness (T''). In general, the length for a tape roll embodiment
is arbitrary, because the length of the tape 320 is substantially
longer than any individual application. Accordingly, lengths of
tape 320 suitable for intended applications can be separated (for
example, "cut") from the roll 330. Again, similar to the previously
described sheet 300, the tape 320 can include a first adhesive
layer 340. The tape 320 can also include a second adhesive layer,
similar to two-sided fastening tape.
Referring now to FIG. 4, an alternative embodiment of a thermal EMI
shield 100 configured as a sheet 400 is illustrated. In this
embodiment, desired application shapes, such as a rectangle 410'
and an ellipse 410'' (generally 410) can be die-cut from the sheet
400, thereby yielding thermal EMI absorbers 100 of any desired
two-dimensional shape. Accordingly, the sheet 400 can be die-cut to
produce the desired outlines of the application shapes 410.
Alternatively, the desired outlines of the application shapes 410
can be custom cut from the blank sheet 300 shown in FIG. 3A.
In yet another embodiment, the thermal EMI shield material may be
preformed in any desired shape. Referring now to FIG. 5, a
preformed shield 500 in a non-planar application is illustrated.
The thermal EMI shield may be molded or extruded in any desired
shape, such as the rectangular trough shown, a cylindrical trough,
and semi-circular trough. Such non-planar thermal EMI shields 500
can be used in connection with non-planar electrical components
200, such as cylindrical devices or components (for example,
"cans").
Referring to FIG. 6, an embodiment of a liquid thermal EMI shield
600 is illustrated. In general, a vessel 610 is shown holding a
liquid thermal EMI shield solution 620. A portion of the solution
620 "A" is illustrated in greater detail in an insert labeled
"Detail View A." The detail view illustrates that the solution 620
includes EMI absorber particles 630 and thermal conductor particles
640, each suspended within a liquid matrix 650. Generally, the
attributes of the particles 630, 640 are similar to the attributes
of the corresponding particles 110, 120 described in relation to
FIG. 1. Similar to the matrix described in relation to FIG. 1, the
liquid matrix 650 is substantially transparent to electromagnetic
radiation. The liquid matrix 650 can be formed as a liquid that may
be painted onto an applicable surface to be treated. Alternatively,
the liquid matrix 650 can be formed as a gel, such as grease, or as
a paste or pour-in-place compound. In some embodiments, the liquid
thermal EMI shield 600 can be applied to the intended surface by
painting, spraying, or other suitable method. The matrix material
may also be a liquid selected from the group consisting of
silicones, epoxies, polyester resins and combinations thereof.
In one embodiment, the matrix 130 illustrated in FIG. 1 is a
suitably selected phase-change material having properties of both a
solid and a liquid. At ambient room temperatures, the phase-change
material behaves as a solid offering ease of handling and storage.
The phase-change material, however, exhibits a reflow temperature
at or below the equipment operating temperature thereby enabling a
"wetting action." The matrix 130 reflows allowing the EMI-absorbing
particles 110 and the thermally conductive particles 120 of the
composite material 100 to flow into any gaps, such as those caused
by surface imperfections.
Referring to FIG. 7, a close-up detail of a cross-sectional view of
an electronic component 700, a heat sink 710, and a thermally
conducting EMI shield 720 is illustrated. Also shown are the
surface imperfections 730 of each or both of the component 700 and
heat sink 710. The surface imperfections 730 are portrayed in an
exaggerated manner for the purpose of illustration. With an ability
to flow into surface imperfections 730, a matrix 650 formulated as
a liquid, or phase-change material removes air gaps, thereby
minimizing the thermal impedance between the device 700 and an
associated heat sink 710. The overall effect of removing air gaps
reduces the thermal impedance between the electrical component 700
and the heat sink 710, leading to improved heat transfer
efficiency. The matrix material may be a mixture of a paraffin wax
having a melting point of approximately 51.degree. C. and a 28%
ethylene-vinyl acetate copolymer having a melting point of
approximately 74.degree. C. For example, a mixture of ninety-five
parts by weight of the paraffin wax and five parts by weight of the
ethylene-vinyl acetate copolymer may be used. Alternatively, a
mixture of twenty-five parts by weight of the paraffin wax and six
parts by weight of the ethylene-vinyl acetate copolymer may be
used. Alternatively still, the matrix material may be a synthetic
wax having a melting point of approximately 100.degree. C. and a
molecular weight of approximately 1000. Such a wax is of a type
known as a Fischer-Tropsch wax.
Referring to FIG. 8, a flow diagram is illustrated depicting a
process of preparing a thermally-conductive EMI absorber 100, such
as the embodiments illustrated in either FIG. 1 or FIG. 6. EMI
absorber particles 110, 630 are provided at step 800. Thermally
conductive particles 120, 640 are also provided at step 810. The
EMI absorber particles 110, 630 and thermally conducting particles
120, 640 are combined and suspended within either a solid matrix
material 130, or a liquid matrix material 650. Once prepared, the
composite thermal EMI shield 100, 600 is applied between an
electronic component 200, 700 and a heat sink 220, 710 at step
830.
Having shown exemplary and preferred embodiments, one skilled in
the art will realize that many variations are possible within the
scope and spirit of the claimed invention. It is therefore the
intention to limit the invention only by the scope of the claims,
including all variants and equivalents.
APPENDIX A
Test Report
Scope:
This report summarizes the thermal conductivity testing of multiple
electromagnetic-energy-absorbing materials including a thermally
conductive filler to also provide good thermal conductivity.
Part Description:
Three test samples were prepared and tested for thermal
performance. Each of the samples consisted of iron (Fe)-filled
elastomeric materials formulated to absorb electromagnetic surface
waves. Some specific details for the test samples are listed below
in Table 1.
TABLE-US-00001 TABLE 1 Test Samples Sample No. Test Sample
Description 1 50% Fe by volume in isoprene, test slab thickness of
30, 60, 90 and 125 mils. 2 41.5% Fe by volume in silicone, test
slab thickness of 20, 30, 60 and 100 mils. 3 40% Fe plus 20%
aluminum nitride (AIN) by volume in silicone, test slab thickness
of 30, 60, 90 and 120 mils.
Test Procedure:
Thermal resistance testing was conducted in accordance within
internal test procedure and in accordance with ASTM specification
D5470. The test samples were first die-cut into 1-inch-diameter
circles to match the size of the thermal impedance probes. All of
the thermal resistance measurements were made at 50.degree. C., and
100 psi.
The test fixture 900 is shown in FIG. 9. The test sample 910 is
placed between two polished metal plates 920, 930 that are stacked
within the test assembly 900 as shown in FIG. 9. The heat is input
from the heater plate 940, which is protected from heat loss in all
directions other than the testing direction by applying the same
temperature to a guard heater 950 that is located above and around
the heater plate 940. An upper meter block 920 is located directly
below the heater 940 and is followed by the test sample 910 find
then a lower meter block 930. Heat is drawn out from the bottom of
the test stack with a water-cooled chiller plate 960. Thermocouples
970 embedded in the meter blocks 920, 930 are used to extrapolate
the surface temperature on each side of the test sample 910. This
is done using a SRM 1462 reference material that has a thermal
conductivity much greater than that of the test sample.
During the test the sample is compressed at a constant pressure
using a pneumatic cylinder. The stack is then permitted to reach a
steady state at which point the thermal resistance of the sample is
calculated. Once the thermal resistance of several thicknesses of
material (nominally five) is measured and plotted the thermal
conductivity is calculated as the inverse of the slope of the least
squares best fit line through this data.
Test Results:
The thermal conductivity of the three absorbing test samples is
shown in Table 2. The two standard absorbing materials, Sample No.
1 and Sample No. 2, have very similar thermal conductivities
(approximately 1.0 Watts/m-.degree. C.), whereas the third absorber
material, Sample No. 3, has a substantially higher thermal
conductivity (approximately 1.5 Watts/m-.degree. C.).
TABLE-US-00002 TABLE 2 Thermal Conductivity Thermal Conductivity
Standard Test Sample (Watts/m-.degree. C.) Deviation Sample No. 1
0.986 0.0632 Sample No. 2 1.022 0.0959 Sample No. 3 1.511
0.0637
TABLE-US-00003 APPENDIX B NITRILE RUBBER (40%) Frequency (GHz)
.mu..sub.r .mu..sub.i .epsilon..sub.r .epsilon..sub.i 0.915 4 -1.77
12.277 -0.251 1.15 4 -1.77 12.277 -0.251 2 3.4 -1.74 12.277 -0.251
2.245 3.29 -1.735 12.277 -0.251 3 2.95 -1.72 12.277 -0.251 4 2.58
-1.67 12.277 -0.251 5 2.219 -1.624 12.277 -0.251 6 2.05 -1.58
12.277 -0.251 7 1.88 -1.55 12.277 -0.251 8 1.65 -1.52 12.277 -0.251
9 1.5 -1.48 12.277 -0.251 9.5 1.45 -1.43 12.277 -0.251 10 1.39 -1.4
12.277 -0.251 11 1.34 -1.36 12.277 -0.251 12 1.27 -1.32 12.277
-0.251 13 1.201 -1.273 12.277 -0.251 14 1.18 -1.24 12.277 -0.251 15
1.14 -1.21 12.277 -0.251 15.5 1.1 -1.18 12.277 -0.251 16 1.057
-1.147 12.277 -0.251 17 1.04 -1.125 12.277 -0.251 18 1.03 -1.1
12.277 -0.251 20 0.854 -0.955 12.277 -0.251 25 0.68 -0.74 12.277
-0.251 30 0.6 -0.54 12.277 -0.251 35 0.533 -0.34 12.277 -0.251 40
0.461 -0.165 12.277 -0.251
* * * * *